1. Field of the Invention
The Government has rights in this invention pursuant to a contract awarded by the Department of Energy.
This invention relates to the field of analytical testing of the chemical composition of a sample. In particular, the present invention relates to a device for performing Electron Spectroscopy for Chemical Analysis (ESCA) and Raman spectroscopic studies.
2. Background of the Invention
The molecular composition of corrosion films and deposits on metal surfaces has been of interest for many years. In particular, it is well known that the life of a power generation plant can be extended if corrosion can be controlled to a point of minimization or elimination. To control corrosion, the corrosion products must first be accurately identified. This information can then be used with supplemental process information to identify and arrest the chemical mechanism from which corrosion products form in a system. Typically, corrosion products on metal surfaces are characterized by a combination of a host of analytical techniques which include, in part, Auger, X-ray Diffraction (XRD), Electron Spectroscopy for Chemical Analysis (ESCA), and most recently, Raman spectroscopy. Each of these analytical techniques provides a limited amount of information and neither technique alone can be used to unambiguously identify the compounds present in a corrosion film or deposit. For example, Auger provides elemental information that requires the analyst to hypothesize a molecular composition which must be confirmed by a secondary technique. XRD provides molecular composition information but is highly sensitive to (1) material concentration (the limit of detection is approximately 2-3%), (2) material composition (the technique cannot detect. amorphous material), and (3) sample geometry (sample face curvature and roughness degrade the spectra). As a result, the XRD technique cannot be used to unambiguously determine the true molecular composition of a corrosion product. The ESCA technique provides direct molecular composition information on all types of materials (including amorphous materials) by measuring molecular field dependent binding energies of atoms, but can not readily distinguish between various oxidization states of some elements such as iron (ESCA cannot accurately distinguish between Fe.sup.2+ and Fe.sup.3+). Because most corrosion products in a power generation plant consist of iron oxides and various doped iron oxides, the ESCA technique can only be used to speculate on the true molecular nature of iron oxide compounds present in a corrosion product from such a system. Finally, Raman spectroscopy provides molecular information on all types of materials (including amorphous materials and glasses) but cannot detect molecules that are not amenable to an internal dipole change (such as Cu.sub.2 S). As a result, it cannot be guaranteed that the Raman technique will detect all compounds present in a corrosion product.
ESCA and Raman spectroscopy synergistically complement each other in the chemical analysis of corrosion products on metal surfaces. In this case, molecular composition analysis information that cannot be obtained by one technique can be obtained by the other to give the most complete and unambiguous analysis of a corrosion product sample. For example, ESCA cannot readily distinguish between Fe.sup.2+ and Fe.sup.3+ based compounds whereas Raman spectroscopy produces well resolved unique vibrational fingerprint. spectra for most Fe.sup.2+ and Fe.sup.3+ corrosion product compounds such as .alpha.-FeOOH, .beta.-FeOOH, .gamma.-FeOOH, Fe(OH).sup.2, Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4. As a second example, ESCA can identify that a material contains phosphorus and oxygen but cannot distinguish between the various types of phosphates such as PO.sub.4.sup.3-, HPO.sub.4.sup.2-, and phosphate from NaFePO.sub.4. Raman, on the other hand, produces clearly defined spectra for these species. Finally, and in contrast, ESCA can identify compounds such as Cu.sub.2 S which cannot be detected by the Raman technique because such symmetrical compounds are not amenable to internal dipole changes which are needed for Raman analyses. Therefore, these two techniques synergistically complement each other and the integration of these two techniques results in a powerful analytical tool what will enable rapid, accurate, and unambiguous identifications of the chemical compositions of corrosion films or deposits in one single analysis without the need to use any other analytical techniques to confirm the results.
It is impractical to perform ESCA and Raman measurements on separate ESCA and Raman instruments because (1) the need to break the high vacuum of the ESCA instrument to transfer the sample to the Raman spectrometer will subject any newly exposed corrosion product to oxidizing room air which will compromise the sample integrity and produce erroneous results, and (2) the inability to accurately position the sample on both instruments so that both techniques are obtaining data from precisely the same location on the specimen.
Accordingly, a need remains for an integrated analytical instrument in which both ESCA and Raman measurements can be performed without exposing samples to air, and without the need for repositioning the sample between ESCA and Raman measurements. An integrated ESCA/Raman analytical instrument is one in which both ESCA and Raman analyses are performed on corrosion products on specimens located in a vacuum chamber. The ESCA/Raman system enables the rapid acquisition of molecular corrosion films and deposits on metal specimens.